3 Answers
3

As nimcap answered, unusual hats are just hats with special particle effects. They have no stat benefits, only visual effects.

The only way to obtain them is to trade for them or to find one in a Mann Co Supply Crate. Crates have a low chance to contain an unusual hat.

According to data gathered by users on the forums, it seems that series 1 crates have about 2.45% chance of containing an unusual item, and series 2 crates have about a 1.5% chance. You can tell which series of crate you have by reading the tooltip.

To open the crate you need to buy a Mann Co. Supply Crate Key, which costs $2.49 in the store. If you'd rather not spend money you can trade the crate to someone. Items that come out of crates are craftable and tradeable.

You can find more details about the drop rate on the TF2 wiki page about crates. Keep in mind that this data is gathered by players and might not be totally accurate, but it's probably in the ballpark.

Unusual hats were added to the game during the Mann-conomy update on September 30th.

These are hats have particle effects attached to them, such as ghosts or a sunbeam. The names for these items are shown in purple. They are not available through the random drop system, but instead as a rare drop from a Mann Co. Supply Crate, which require you to buy keys from the Mann Co. Store (for $2.49 each) to open. Consult the list below to know which effects are dropped from which crates.

Of course, like all hats, they can be traded. Expect to trade a lot of keys/earbuds for one, though, as they are exceedingly rare.

If you are trading to try and get Unusual hats, note that items can now be renamed... check the name color (should be purple for Unusual items) to make sure it really is an Unusual hat.

Also, there have been scammers out there lately who will put in an item and then change it while you are putting your items into the trade window. Always double-check what items are in the other person's side of the trade window before accepting.

The list of available Unusual hat effects are:

Series #1 (Crates #1-25)

Haunted Ghosts

Circling TF Logo

Circling Heart

Circling Peace Sign

Sunbeam

Massed Flies

Green Confetti

Purple Confetti

Green Energy

Purple Energy

Burning Flames

Scorching Flames

Searing Plasma

Vivid Plasma

Series #2 (Crates #26-57)

Stormy Storm

Blizzardy Storm

Bubbling

Smoking

Steaming

Nuts n' Bolts

Orbiting Planets

Orbiting Fire

Series #3 (Crates #59-91 (except #87))

Aces High

Disco Beatdown

Miami Nights

Dead Presidents

Cloud 9

Kill-a-Watt

Terror-Watt

Robo (Crate #58 - Robo Community Crate)

Anti-Freeze

Roboactive

Memory Leak

Phosphorous

Sulphurous

Overclocked

Power Surge

Electrostatic

Green Black Hole

Time Warp

End of the Line (Crate #87 - End of the Line Community Crate)

Death At Dusk

Morning Glory

Frostbite

Molten Mallard

Taunts (Mann Co. Audition Reel, Mann Co. Director's Cut Reel, End of the Line Community Crate)

'72

Fountain of Delight

Holy Grail

Mega Strike

Midnight Whirlwind

Silver Cyclone

Screaming Tiger

Skill Gotten Gains

Showstopper

Halloween (All normal crates during the Halloween Event) Number in parentheses is the year the effect was added.